I may not be the best person to answer this as I work for Hitachi Vantara Global support so the bias is obviously leaning towards our support. That being said you did not mention what kind of support you require on which platforms and to what extent. There may be reasons you want to go with a third party company if you have for example equipment from multiple vendors but still want to have a single support-bill and contact point.

When it comes to technical support be aware that NO THIRD PARTY company has direct access to any engineering groups of particular vendors. They still need to contact the individual support organizations of each of those vendors and any update, bugfix etc will trickle down in the same way.

In all cases it is up to you the customer to make that decision. There are really smart guys out there working for Hitachi partners who a very capable in helping out in many situations. When it comes to specific analysis of equipment behaviour you are in most cases depending on the expertise of the vendor support folks.